This invention relates to printers and, in particular, to a method for accurately reproducing a color image by a color printer.
When reproducing a computer monitor""s displayed color image by a color printer, the user typically wants the exact same colors the user perceives on the monitor to be printed on white paper. The human eye adapts a viewed color to a xe2x80x9cwhitexe2x80x9d reference on the medium displaying the color. This is referred to as chromatic adaptation. However, the human eye does not completely adapt to the white point of the monitor.
The monitor white xe2x80x9ccolorxe2x80x9d is generally not the same white as the paper viewed under standard office light. For example, the white point on a monitor screen may have certain red, green, or blue components, while the white point of paper is generally that of the viewing illuminant having color components different from that of the monitor white point. Therefore, since the white references on the monitor and the paper are different, simply converting the RGB primary color components used to form the monitor colors to the corresponding cyan, magenta, and yellow (CMY) primary color ink components used in a color printer will not accurately reproduce (to the human eye) the colors perceived by the user when viewing the monitor.
An additional factor affects the viewer""s perception of color. Printed color on white paper is usually viewed in a well lit environment, causing the white paper to be bright. The monitor screen is typically viewed in a darker area, giving the perception that the monitor white point is fairly bright. If the monitor white and paper white were viewed under the same illumination, the monitor white would appear less bright. This would further distance the monitor white from the paper white.
Accordingly, monitor white may not only contain color components that are different from the components in the paper white but may also be darker than the paper white.
As seen, due to chromatic adaptation, simply converting the RGB monitor signals into the corresponding CMY printer signals do not reproduce the colors on the monitor as viewed by the human eye.
To compensate for the chromatic adaptation, prior solutions have included shifting the monitor RGB color signals. One approach is to assume full chromatic adaptation and then determine the shift in colors to cause the monitor white point to be the same as the paper white point (assuming a particular viewing illuminant.) One problem with this approach is that full chromatic adaptation of the human eye does not occur.
Another problem discovered by the Applicants with prior art solutions is that shifting the monitor colors also shifts neutral colors (i.e., gray scale colors.) A user generating gray scale colors on a monitor does not wish these gray scale colors to contain any CMY colors when printed. Rather, the user would want the gray scale to be printed using a black dot halftone image.
Hence, what is needed is an improved technique for accurately reproducing monitor colors by a color printer without shifting the neutral axis.
In one embodiment, the process includes transforming non-neutral monitor colors to generate transformed monitor colors, such that printing the transformed monitor colors using a color printer causes printed colors to appear to the human eye to be similar to the monitor colors displayed on a monitor, while preserving neutral colors, once the human eye has adapted to the monitor colors and the printed colors.
When a monitor display is to be printed by a color printer, the present invention changes the monitor colors to compensate for chromatic adaptation while mapping the monitor neutral axis precisely with the printer""s neutral axis so neutral colors remain neutral.
In the preferred embodiment, it is assumed that there is only partial chromatic adaptation by the viewer.